Article made by joining two members together, and a brazing filler metal

ABSTRACT

In an article, a binding layer formed through a heating step exists between a permanent magnet and a laminate of steel plates. The binding layer is formed by producing a liquid phase at the heating step from a brazing filler metal made of a rare earth element-based alloy. Diffusion areas exist between a body portion of the permanent magnet and a body portion of the binding layer and between a body portion of the laminate and the body portion of the binding layer, respectively. The thickness t of each of the diffusion areas is in a range of t≧5 μm. The bond strength in the article is greatly increased by the provision of the diffusion areas.

This is a Continuation of Ser. No. 08/480,269, filed Jun. 7, 1995, now U.S. Pat. No. 5,830,585.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an article made by joining two members together, and particularly, to an article containing two members bound to each other with a binding layer formed through a heating step between one of the members made of a metal and the other member made of a material of the same type or a different type as the one member. The present invention also relates to a brazing filler metal for forming that binding layer.

2. Description of the Prior Art

A rotor for a motor is a conventionally known article of this type. In this rotor, one member is a permanent magnet containing a rare earth element, and the other member is a rotor body made of a steel which is a different material than the permanent magnet.

The permanent magnet containing the rare earth element has a nature that the machinability thereof is poor, because the permanent magnet is very brittle, and that if the permanent magnet is exposed to a high temperature, the metallographic structure thereof is changed, and as a result, the magnetic characteristic of the permanent magnet is influenced.

In mounting the permanent magnet to the steel rotor body, a mounting means such as a dovetail structure, a screwing, a welding and the like cannot be employed and hence, an adhesive has conventionally been employed.

However, if an adhesive is used, the following problems are encountered: the strength of bonding between the permanent magnet and the rotor body is low, because of a poor wettability of the permanent magnet, and the bonding strength is significantly reduced with a rise in temperature. In such a situation, it is not possible to meet the demand for an increase in speed of rotation of the motor.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an article in which the strength of bonding between two members is increased.

To achieve the above object, according to the present invention, there is provided an article made by joining two members together, including a binding layer formed through a heating step between one of the members which is made of a metal and the other member which is made of the same material or a different material as the one member, wherein the binding layer is formed by production of a liquid phase in the heating step from a brazing filler metal of a rare earth element-based alloy, and the article further includes diffusion areas existing between the one member and the binding layer and between the other members and the binding layer, respectively, the thickness t of each of the diffusion areas being in a range of t≧5 μm.

The liquid phase produced from the rare earth element-based alloy is highly active and exhibits an excellent wettability to various types of members to be bound, thereby inducing a mutual diffusion between these members to be bound, which is accompanied by an active migration of atoms.

In this manner, the members are bound with the binding layer in a diffusion fashion and therefore, the strength of bonding between the members is greatly enhanced.

However, if the thickness t of each diffusion area is smaller than 5 μm, the strength of bonding between both the members is decreased. The upper limit value of the thickness t of the diffusion area is substantially equal to the thickness of the brazing filler metal used. Therefore, the entire binding layer may be a diffusion area in some cases.

The above objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an article made by joining two members;

FIG. 2 is an enlarged sectional view of an essential portion of the article;

FIG. 3 is a graph illustrating the relationship between the temperature and the thermal expansion coefficient;

FIG. 4 shows an essential portion of a Cu—Nd state diagram;

FIG. 5 is a perspective view illustrating a permanent magnet, a brazing filler metal and a laminate in a superposed relation;

FIG. 6 is a perspective view of a sandwich structure;

FIG. 7 is a photomicrograph showing the metallographic structure of a bonded area in the sandwich structure;

FIGS. 8A and 8B are graphs illustrating the results of a linear analysis for the binding area in the sandwich structure;

FIG. 9 is a graph illustrating the relationship between the thickness t of a diffusion area and the tensile strength;

FIG. 10 is an X-ray diffraction pattern for Nd₇₀Cu₂₅Al₅;

FIG. 11 is a photomicrograph showing the metallographic structure of the binding area in the sandwich structure;

FIGS. 12A, 12B and 12C are graphs illustrating the results of a linear analysis for the binding area in the sandwich structure;

FIGS. 13A and 13B are graphs illustrating the results of a linear analysis for the binding area in the sandwich structure;

FIG. 14 is a perspective view of an article;

FIG. 15 is a graph illustrating the result of a linear analysis for the binding area in the sandwich structure;

FIG. 16 is a graph illustrating the relationship between the Cr content and the tensile strength;

FIG. 17 is a sectional view of a rotor for a motor, taken along a line 17—17 in FIG. 18;

FIG. 18 is a sectional view taken along a line 18—18 in FIG. 17;

FIG. 19 is an X-ray diffraction pattern for a thin foil and an ingot;

FIG. 20 is an X-ray diffraction pattern for a Nd₇₀Cu₂₀C₁₀ alloy;

FIG. 21 is a diagram illustrating the composition of a Nd—Cu—Al based alloy; and

FIG. 22 is an X-ray diffraction pattern for a Pr₆₈Cu₂₇Al₅ alloy;

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a preferred embodiment of an article 1. In the article 1, one member made of a metal is a permanent magnet 2 such as an NdFeB based permanent magnet, an SmCo based permanent magnet and the like, and another member made of the same or different type of material as the permanent magnet 2 is a laminate 5 made by laminating cold rolled steel plates made of an Fe-based alloy one on another and integrally coupling them by a crimping means.

A binding layer 6 formed through a heating step exists between the permanent magnet 2 and the laminate 5. The binding layer 6 is formed by the production of a liquid phase at the heating step of a brazing filler metal made of a rare earth element-based alloy.

As shown in FIG. 2, diffusion areas 20 and 22 exist between the permanent magnet 2 and the binding layer 6 and between the laminate 5 and the binding layer 6, respectively. In the embodiment, diffusion areas 10 and 11 exist between a body portion 7 of the permanent magnet 2 and a body portion 8 of the binding layer 6 and between a body portion 9 of the laminate 5 and the body portion 8 of the binding layer 6, respectively. The thickness t of each of the diffusion areas 10 and 11 is set in a range of t≧5 μm. In FIG. 2, reference character “a” designates a binding surface of the permanent magnet 2, and reference character “b” designates a binding surface of the laminate 5. The binding surface b is formed from end faces of the steel plates 3.

The rare earth element alloy forming the binding layer 6, namely, the brazing filler metal comprises a rare earth element which is basically a predominant constituent, and an alloy element AE which performs a eutectic reaction with the rare earth element. The rare earth element is at least one element selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb an Lu. They may be used alone or in the form of Mm (Misch metal) or Di (didymium). The alloy element AE is at least one element selected from the group consisting of Cu, Al, Ga, Co, Fe, Ag, Ni, Au, Mn, Zn, Pd, Sn, Sb, Pb, Bi, Ge and In. The content of the alloy element AE is set in a range of 5 atom %≦AE≦50 atom %.

If the content of the alloy element AE in the brazing filler metal is lower than 5 atom %, or higher than 50 atom %, the volume fraction Vf of the liquid phase in a solid-liquid coexisting state is decreased, resulting in a reduced bond strength. As a result, it is desirable that the content of the alloy element AE is set so as to provide a eutectic composition or a composition near the eutectic composition in the relationship with the rare earth element.

When the brazing material contains two or more alloy elements, the total content of the alloy elements is in a range of 5 atom %≦AE≦50 atom %.

The brazing filler metal may contain at least one of Cr and V as a diffusion area forming element DA in a range of DA≦5 atom %. If the content DA of the diffusion area forming element DA is greater than 5 atom %, the diffusion area forming element DA forms a brittle intermetallic compound in the diffusion areas 10 and 11, resulting in a significantly reduced bond strength. If even a small amount of the diffusion area forming element DA is contained in the brazing filler metal, an effect of the diffusion area forming element DA is revealed, but the lower limit value of the content of the diffusion area forming element DA preferably is set at 0.1 atom %, so that the diffusion area is reliably formed.

When the permanent magnet 2 is, for example, an NdFeB based permanent magnet, the permanent magnet 2 shows little elongation and has a longitudinal elastic modulus E nearly equal to 16,000 kgf/mm². The permanent magnet 2 also has a characteristic that its thermal expansion coefficient has a reverse turn, i.e., starts to increase at about 310° C. If such an NdFeB based permanent magnet is bound in a heating manner by a brazing filler metal having a high longitudinal elastic modulus E, there is a possibility that the permanent magnet may be broken by a thermal stress within the permanent magnet at room temperature after binding.

If a member to be bound having a brittleness as described above is assumed, it is desirable that the longitudinal elastic modulus E of the brazing filler metal is in a range of E≦10,000 kgf/mm². In this case, the lower limit value of the longitudinal elastic modulus E is set at 300 kgf/mm², so that the rigidity of the binding layer 6 during rotation is insured.

The rare earth element based eutectic alloys as the brazing filler metal are exemplified in Tables 1 and 2.

TABLE 1 Rare earth Longitudinal element Alloy element Eutectic elastic modulus E (atom %) (atom % ) point (° C.) (kgf/mm²) Nd 88 Sn 12 870 3850 88 In 12 880 3850 85 Al 15 635 3900 80 Ag 20 640 4400 80 Ga 20 651 4300 77 Zn 23 630 4400 75 Fe 25 640 4500 75 Mn 25 700 4400 70 Cu 30 520 4040 64 Co 36 599 5000 Ce 84 Au 16 520 3700 84 Mn 16 620 4000 82 Ni 18 470 3800 65.5 34.5 453 5100 81.75 Ga 18.25 495 4300 72 Cu 28 424 4100

TABLE 2 Rare earth element Alloy element Eutectic Longitudinal elastic (atom % (atom % ) point (° C.) modulus E (kgf/mm²) La 95 Sb 5 845 3900 90 Sn 10 716 3900 86.5 In 13.5 742 3900 83 Pb 17 495 4000 80 Ga 20 550 4100 80 Co 20 520 4000 69 31 541 4200 88 Ge 12 730 4400 75 Zn 25 530 4100 71 Cu 29 475 4000 69.6 Bi 30.4 820 4100 Sm 80 Pd 20 692 3950 71 Cu 29 597 4050 Pr 80.5 Co 19.5 570 4000 66 34 541 4500 68 Cu 32 472 5000 Ho 73 Mn 27 875 5100

Examples of rare earth element-based hypoutectic alloys and rare earth element-based over-eutectic alloys are Nd₆₀Cu₄₀ alloy (E=4,500 kgf/mm²), Nd₇₆Cu₂₈ alloy (E=4,000 kgf/mm²), Nd₈₀Cu₂₀ alloy (E=3,950 kgf/mm²), Nd₅₀Cu₃₀ alloy (E=9,000 kgf/mm²), Nd₉₀Al₁₀ alloy (E=3,850 kgf/mm²), Nd₈₀Co₂₀ alloy (E=4,000 kgf/mm²), Sm₇₅Cu₂₅ alloy (E=4,000 kgf/mm²), Sm₆₅Cu₃₅ alloy (E=4,300 kgf/mm²), La₈₅Ga₁₅ alloy (E=4,000 kgf/mm²), and the like, wherein the unit of numerical values in each of the chemical formulae is atom %, and E means the longitudinal elastic modulus. Further, examples of ternary alloys are Nd₆₅Fe₅Cu₃₀ alloy (having a liquid phase generating temperature of 501° C. and E=4,200 kgf/mm²), Nd₇₀Cu₂₅Al₅ alloy (having a liquid phase generating temperature of 474° C. and E=4,000 kgf/mm²), and the like.

The desirable brazing filler metals are those formed from rare earth element based alloys having a volume fraction Vf of an amorphous phase in a range of Vf≧50% (including Vf of 100%).

One method for producing such a rare earth element-based alloy, a liquid quenching process, e.g., a single-roller melt-spinning technique is preferred. More specifically, (a) A molten metal having a rare earth element-based alloy composition is prepared. (b) The molten metal is ejected onto an outer peripheral surface of a cooling roll made of Cu and rotating at a high speed and subjected to a ultra-quenching, thereby providing a thin foil (filler) having a volume fraction Vf of an amorphous phase equal to or greater than 50%. The brazing filler metal is cut off this thin foil.

The rare earth element-based alloy has a composition such that it has a content of a rare earth element RE is in a range of RE≧50 atom %, a Cu content in a range of 20 atom %≦Cu≦40 atom %, and a content of another alloy element AE in a range of AE≦20 atom %. In addition, the rare earth element RE is at least one element selected from the group consisting of La, Ce, Pr, Nd an Sm, and the alloy element AE is at least one element selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Ag, Au, Zn, B, Al, Ga, In, C, Si, Ge, Sn, Pb, P, Sb and Bi.

If the contents of the rare earth element RE, Cu and the alloy element AE are specified in the above manner, the rare earth element RE, Cu and the alloy element AE perform a eutectic reaction under heating and hence, the temperature Tm for generating the liquid phase of the rare earth-based alloy is relatively low. This achieves a fusibilization.

The rare earth element-based alloy containing Cu is non-crystallized, i.e., rendered amorphous using a liquid quenching process. If the selected alloy element is contained in a particular amount in this rare earth element-based alloy, the non-crystallization of the rare earth element-based alloy is promoted, and the continuity in forming a thin foil is improved. Therefore, a thin foil of an amorphous alloy having a thickness of about 10 to about 150 μm and being uniform can easily be produced utilizing a single-roller melt-spinning technique.

Such a thin foil is of a high toughness. For this reason, when the thin foil is used as a brazing filler metal, the thin foil can be die-punched into a desired shape. In addition, because the thin foil is extremely thin, the adjustment of the thickness to a value necessary for the brazing filler metal can be achieved by simply laminating a foil to the thin foil. Moreover, the thin foil also has an advantage that because a grain boundary layer likely to become a start point of oxidation does not exist in the thin foil and hence, the thin foil has an excellent oxidation resistance, and an oxide as an impurity is not incorporated in the binding process.

The thickness of the thin foil is adjusted by the peripheral speed of the cooling roll, the ejection pressure of the molten metal, the temperature of the molten metal and the like. The thickness of the thin foil is ½ to {fraction (1/10)} of the thickness of the brazing filler metal cut off an ingot.

However, if the content of the rare earth element RE in the rare earth element-based alloy is smaller than 50 atom %, the liquid phase generating temperature Tm tends to be increased. If such rare earth element-based alloy is used as a brazing filler metal, the bond strength is lowered. If the Cu content is smaller than 20 atom %, it is difficult to achieve the non-crystallization. On the other hand, if Cu≧40 atom %, the liquid phase generating temperature Tm is increased. If the content of the alloy element AE is larger than 20 atom %, the continuity and uniformization of the thin foil made of the amorphous alloy are degraded.

In binding the permanent magnet 2 with the laminate 5, a procedure is employed which involves superposing both the members 2 and 5 one on another with a brazing filler metal in the form of a thin plate, a foil piece or the like interposed therebetween, placing the superpositioned block into a vacuum heating furnace, heating the superpositioned block to bring the brazing filler metal into a liquid phase state or a solid-liquid coexisting state, and cooling the superpositioned mass.

In this case, the heating temperature T is varied depending upon the composition of the brazing filler metal, but each of the various brazing filler metals having the above-described compositions is brought into the liquid state or the solid-liquid coexisting state at a relatively low heating temperature T. Because the magnetic characteristic of the permanent magnet 2 is influenced by the heating temperature, a brazing filler metal should be selected so as to avoid the variation in magnetic characteristic.

The liquid phase produced from the brazing filler metal, i.e., the rare earth element-based alloy is highly active and exhibits an excellent wettability to various members to be bound, namely the permanent magnet 2 and the laminate 5 to induce the mutual diffusion between the permanent magnet 2 and the laminate 5, which is accompanied by an active migration of the atoms.

In this manner, the permanent magnet 2, the laminate 5 and the binding layer 6 are bound together in a diffusion fashion and hence, the strength of the bond between the permanent magnet 2 and the laminate 5 is greatly enhanced.

Further, it is possible to avoid the fracture of the NdFeB based permanent magnet 2 after being bound, by setting the longitudinal elastic modulus E of the brazing filler metal in the above-described manner.

If the heating time h is too long, the characteristic of the permanent magnet 2 or the like is influenced by such too long heating. Therefore, it is desirable that the heating time h is equal to or shorter than 10 hours, but from the viewpoint of an enhancement in productivity, the heating time h is equal to or shorter than 1 hour.

The magnetizing treatment for the permanent magnet 2 is carried out after the binding process.

Example 1

Neodymium (Nd) having a purity of 99.9% and copper (Cu) having a purity of 99.9% were weighed, so that a Nd₇₀Cu₃₀ alloy having a eutectic-point composition was produced. Then, the weighed elements were melted using a vacuum melting furnace, and then subjected to a casting process to produce an ingot having a length of 10 mm, a width of 10 mm and a height of 50 mm. The ingot was subjected to a cutting by a micro-cutter to fabricate a thin plate-like brazing filler metal made of the Nd₇₀Cu₃₀ alloy and having a length of 10 mm, a width of 10 mm and a thickness of 0.3 mm (see thin plates 12 in FIG. 5). As shown in FIG. 4, the eutectic point of the Nd₇₀Cu₃₀ alloy is of 520° C.

An NdFeB based permanent magnet 2 (made by Sumitomo Special Metals Co., LTD, under a trade name of NEOMAX-28UH and having a Curie point of 310° C.) having a length of 10 mm, a width of 10 mm and a thickness of 5 mm was selected as one member to be bound, while a laminate 5 made by laminating cold rolled carbon steel sheets each having a thickness of 0.4 mm together and having a length of 10 mm, a width of 10 mm and a height of 15 mm was selected as the other member to be bound.

As shown in FIG. 5, a brazing filler metal 12 was superposed on an upwardly facing binding surface b of one laminate 5. A permanent magnet 2 with one binding surface a facing downwardly was superposed on the brazing filler metal. Another brazing filler metal 12 was superposed on the other upwardly facing binding surface a of the permanent magnet 2, and another laminate 5 with its binding surface b facing downwardly was superposed on the brazing filler metal 12, thereby fabricating a superpositioned block. Twenty superpositioned blocks in total were fabricated in the same manner. Then, the superpositioned blocks were placed into a vacuum heating furnace and subjected to a binding process including a heating step at a heating temperature T equal to 530° C. for a heating time h equal to 15 minutes and a subsequent cooling step, thereby producing twenty sandwich structures A having the tow laminates 5 and the permanent magnet 2 bound to each other with the binding layers 6 interposed therebetween, as shown in FIG. 6. Each of the sandwich structures A comprises the two articles 1 commonly associated with the single permanent magnet 2. In this binding process, the heating temperature T was equal to 530° C. and exceeded a liquid phase generating temperature Tm of 520° C. which is the eutectic point and therefore, the brazing filler metal 12 was brought into a liquid phase, because it had the eutectic composition. In this case, no fracture was produced in the permanent magnet 2, because the longitudinal elastic modulus E of the brazing filler metal, i.e., the Nd₇₀Cu₃₀ alloy was equal to 4040 kgf/mm², as given in Table 1 and thus, was ≦10,000 kgf/mm². Through-holes 13 existing in the laminates 5 are used for connection of the sandwich structure with a chuck in a tensile test.

FIG. 7 is a photomicrograph (a reflection electron composition image photograph in a visual field of an analysis) showing the metallographic structure of the binding area in the sandwich structure. It can be seen from FIG. 7 that the permanent magnet 2 and the laminate 5 were tightly bound to each other with the binding layer 6 interposed therebetween.

FIGS. 8A and 8B show the results of a line analysis in a line connecting points X and Y in FIG. 7. As is apparent from these Figures, gradients of concentrations of Fe and Nd atoms exist in the binding area between the permanent magnet 2 and the binding layer 6 and from this, it can be seen that the active mutual diffusion occurred. In this case, the thickness t of the diffusion area 10 is nearly equal to 10 μm. Gradients of concentrations of Fe and Nd atoms also exist in the binding area between the laminate 5 and binding layer 6 and from this, it can be seen that the active mutual diffusion occurred. In this case, the thickness t of the diffusion area 11 is nearly equal to 20 μm.

For comprising, a permanent magnet 2 similar to the above-described permanent magnet 2 and two laminates 5 similar to those described above were superposed one on another by using an epoxy resin based adhesive (made by Nippon CIBA-GEIGY CO., LTD, under a trade name of ARALDITE) to fabricate a superpositioned block similar to the above-described superpositioned block. Twenty superpositioned blocks in total were fabricated in the same manner. Then, the superpositioned blocks were placed into a drying oven and subjected to a binding process involving a heating step at a heating temperature of 200° C. for a heating time of 60 minutes and a cooling step, whereby twenty sandwich structures B (not shown) similar to the above-described sandwich structures and having both of the laminates 5 bound to the permanent magnet 2 by the epoxy resin-based adhesive.

Ten sandwich structures A made using the brazing filler metal 12 and ten sandwich structures B made using the epoxy resin-based adhesive were subjected to a tensile test at room temperature, and the remaining ten sandwich structures of each type were subjected to a tensile test under heating at 150° C., thereby providing the results shown in Table 3. The tensile strengths reported in Table 3 the average values of the tests. This is also true of subsequent Tables and drawings.

TABLE 3 Tensile strength (kgf/mm²) Sandwich Room Standard Standard structure temperature deviation 150° C. deviation A (made using 3.1 0.4 3.0 0.4 brazing filler metal) B (made using 1.5 0.9 0.5 0.2 adhesive)

As is apparent from Table 3, the sandwich structure A made using the brazing filler metal 12 has a high bond strength both at room temperature and under heating at 150° C., as compared with the sandwich structure B made using the epoxy resin-based adhesive, due to the presence of the diffusion areas 10 and 11. The bond strength is little changed under both of the environments, and the change is only slight. Under heating at 150° C., the bond strength of the sandwich structure B is decreased down to ⅓ of the bond strength at room temperature.

The permanent magnet 2 containing a rare earth element such as NdFeB based permanent magnet, SmCo based permanent magnet and the like tends to be reduced in its magnetic characteristic and particularly in coercive form _(L)H_(C) (magnetization intensity I=0) after magnetization, when the heating temperature T in the binding process is higher than 650° C. However, the residual magnetic flux density Br and the coercive force _(B)H_(C) (magnetic flux density B=0) are little changed and hence, the maximum magnetic energy product (BH)max is substantially constant. In the binding process using the brazing filler metal 12, the heating temperature T for the brazing filler metal 12 is equal to 530° C. and thus, is ≦650° C. and therefore, the magnetic characteristic of the permanent magnet 2 cannot be influenced.

The poorness in wettability of the permanent magnet 2 is due to a phase having a high concentration of the rare earth element, e.g., a high concentration of Nd in the embodiment existing at a crystal gain boundary. In the binding process using the brazing filler metal 12, the brazing filler metal 12 was brought into the liquid phase, and the liquid phase produced from the Nd₇₀Cu₃₀ alloy containing Nd as a predominant constituent exhibits an excellent wettability to the permanent magnet 2, because it is highly active and common in the predominant constituent to the phase existing at the crystal grain boundary and having the high Nd concentration. In addition, the liquid phase has an extremely good wettability to the laminate 5 made of the steel plates as a result of an increase in activity. Thus, the formation of the diffusion areas 10 and 11 is easily and reliably performed.

For the purpose of examining the relationship between the thickness t of the diffusion areas 10 and 11 and the bond strength, various sandwich structures A with the diffusion areas 10 and 11 having different thicknesses t were fabricated in the same manner as that described above, and subjected to a tensile test at room temperature.

FIG. 9 shows the results of those tensile tests. As is apparent from FIG. 9, if the thickness t of the diffusion areas 10 and 11 is set in a range of t≦5 μm, the bond strength in the sandwich structure A can be reliably enhanced, as compared with that in the sandwich structure B. Preferably, the thickness t of the diffusion areas 10 and 11 is in a range of t≦10 μm.

Example 2

Neodymium (Nd) having a purity of 99.9%, copper (Cu) having a purity of 99.9% and aluminum having a purity of 99.9% were weighed, so that a Nd₇₀Cu₂₅Al₅ alloy was produced. Then, the weighed elements were melted using a vacuum melting furnace, and then subjected to a casting process to produce an ingot.

About 50 g of a starting material was taken from the ingot and melted by a high frequency within a quartz nozzle to prepare a molten metal. Then, the molten metal was ejected through a slit in the quartz nozzle onto an outer peripheral surface of a cooling roll made of copper (Cu) and rotating at a high speed below the slit under an argon gas pressure, and then subjected to a ultra-quenching, thereby providing a thin foil of the Nd₇₀Cu₂₅Al₅ alloy having a width of 30 mm and a thickness of 100 μm. The thin foil had a metallic sheen.

In this case, the producing conditions were as follows. The inside diameter of the quartz nozzle was 40 mm; the size of the slit is 0.25 mm wide and 30 mm long; the argon gas pressure was 1.0 kgf/cm²; the temperature of the molten metal was 670° C.; the distance between the slit and the cooling roll was 1.0 mm; the peripheral speed of the cooling roll was 13 m/sec; and the cooling rate of the molten metal was about 10⁵ K/sec.

FIG. 10 shows the results of an X-ray diffraction for the thin foil. In the thin foil, a wide halo pattern was observed at 2 θ˜32° and from this, it was determined that the metallographic structure of the thin foil was an amorphous single-layer structure having a volume fraction Vf of an amorphous phase equal to 100%. In addition, the thin foil had a high toughness and was capable of being contact-bent through 180°.

Then, an amorphous foil-like brazing filler metal 12 having a length of 10 mm, a width of 10 mm and a thickness 100 μm was cut off the amorphous thin foil, and using this brazing filler metal 12, a binding operation was carried out in the following manner.

A NdFeB based permanent magnet (made by Sumitomo Special Metals Co., LTD, under a trade name of NEOMAX-28UH and having a Curie point of 310° C.) having a length of 10 mm, a width of 10 mm and a thickness of 5 mm was selected as one member to be bound, as in Example 1, while a rectangular parallelepiped-shaped laminate made by laminating cold rolled carbon steel sheets 3 with a thickness of 0.4 mm together and having a length of 10 mm, a width of 10 mm and a height of 15 mm was selected as the other member to be bound, as in Example 1.

As shown in FIG. 5, a brazing filler metal 12 was superposed on an upwardly facing binding surface b of one laminate 5. A permanent magnet 2 with one binding surface a facing downwardly was superposed on the brazing filler metal 12. Another brazing filler metal 12 was superposed on the other upwardly facing binding surface a of the permanent magnet 2, and another laminate 5 with its binding surface b facing downwardly was superposed on the brazing filler metal 12, thereby fabricating a superpositioned block. Twenty superpositioned blocks in total were fabricated in the same manner. Then, the superpositioned blocks were placed into a vacuum heating furnace and subjected to a heating step at a heating temperature T equal to 530° C. for a heating time h equal to 15 minutes and a subsequent cooling step, thereby producing twenty sandwich structures A having both of the laminates 5 bound to the permanent magnet 2 with the amorphous binding layer 6 interposed therebetween, as shown in FIG. 6. In this binding process, the heating temperature T was equal to 530° C. and exceeded a liquid phase generating temperature Tm of 474° C. for the brazing filler metal 12 and therefore, the brazing filler metal 12 was brought into a liquid phase. In this case, no fracture was produced in the permanent magnet 2, because the longitudinal elastic modulus E of the brazing filler metal, i.e., the Nd₇₀Cu₂₅Al₅ alloy was equal to 4000 kgf/mm², as described above.

FIG. 11 is a photomicrograph (a reflection electron composition image photograph in a visual field of an analysis) showing the metallographic structure of the binding area in the sandwich structure A. It can be seen from FIG. 11 that the permanent magnet 2 and the laminate 5 were tightly bound to each other with the binding layer 6 interposed therebetween.

FIGS. 12A, 12B and 12C show the results of a line analysis in a line connecting points X and Y in FIG. 11. As is apparent from these Figures, gradients of concentrations of Fe, Nd and Al atoms exist in the binding area between the permanent magnet 2 and the binding layer 6 and from this, it can be seen that the active mutual diffusion occurred. In this case, the thickness t of the diffusion area 10 is nearly equal to 8 μm. Gradients of concentrations of Fe, Nd and Al atoms also exist in the binding area between the laminate 5 and binding layer 6 and from this, it can be seen that the active mutual diffusion occurred. In this case, the thickness t of the diffusion area 11 is nearly equal to 25 μm.

Ten sandwich structures A were subjected to a tensile test at room temperature, and the remaining ten sandwich structures A were subjected to a tensile test at a temperature of 150° C., thereby providing the results given in Table 4. For comparison, data for the sandwich structure B made using an epoxy resin-based adhesive are also given in Table 4.

TABLE 4 Tensile strength (kgf/mm²) Sandwich Room Standard Standard structure temperature deviation 150° C. deviation A (made using 4.3 0.4 4.4 0.4 brazing filler metal) B (made using 1.5 0.9 0.5 0.2 adhesive)

As is apparent from Table 4, the sandwich structure A made using the brazing filler metal 12 has a high bond strength at room temperature and under heating at 150° C., as compared with the sandwich structure B made using the epoxy resin-based adhesive, due to the presence of the diffusion areas 10 and 11. The bond strength is little changed under both of the environments, and the change is slight. It also contributes to an enhancement in bond strength that the amorphous brazing filler metal 12 has an excellent oxidation resistance and is homogeneous, and that there is no oxide incorporated in the brazing filler metal 12.

For comparison, a thin foil made of an amorphous Nd₇₀Cu₂₀Al₁₀ alloy having a width of 30 mm and thickness of 100 μm was produced in the same manner as that described above. Then, a foil-like amorphous brazing filler metal having a length of 10 mm, a width of 10 mm was cut off the thin foil and using this brazing filler metal, ten sandwich structures similar to the sandwich structure A shown in FIG. 6 were produced in the same manner as that described above. In this case, the liquid phase generating temperature Tm of the Nd₇₀Cu₂₀Al₁₀ alloy was equal to 473° C., but in the binding process, the heating temperature T was set at 415° C., and the heating time h was set at 120 minutes. Therefore, the brazing filler metal of the Nd₇₀Cu₂₀Al₁₀ alloy was left in a solid phase state.

FIGS. 13A and 13B show the results of a line analysis for the sandwich structure as the comparative example. As obvious from these Figures, gradients of concentrations of Fe and Nd atoms exist in the binding area between the permanent magnet and the binding layer and in the binding area between the laminate and the binding layer. The thickness t of each of the diffusion areas in this case is in a range of t≦2 μm.

The ten sandwich structures as the comparative example were subjected to a tensile test at room temperature to provide results given in Table 5

TABLE 5 Tensile strength (kgf/mm²) Room temperature Standard deviation Sandwich structure 0.5 0.3 as comparative example

In the sandwich structure as the comparative example, it is believed that the solid phase diffusion occurred in the absence of a liquid phase of the brazing filler metal. However, if the solid phase diffusion occurs, the thickness t of the diffusion area is as small as 2 μm or less, resulting in an extremely reduced bond strength, as shown in Table 5.

Example 3

An amorphous foil-like brazing filler metal having a length of 6 mm, a width of 20 mm and a thickness of 100 μm was cut off the amorphous thin foil of Nd₇₀Cu₂₅Al₅ described in Example 2.

Two NdFeB based permanent magnets (made by Sumitomo Special Metals Co., LTD, under a trade name of NEOMAX-28UH and having a Curie point of 310° C.) having a length of 6 mm, a width of 20 mm and a height of 50 mm were selected as the same type of two members to be bound.

As shown in FIG. 14, the brazing filler metal 12 was sandwiched between the end faces of the two permanent magnets 2, and this state was maintained using a preselected jig. Then, the permanent magnets 2 and the brazing filler metal 12 were placed into a vacuum heating furnace and subjected to a heating step at a heating temperature of 520° C. for a heating time of 20 minutes and then to a cooling step, thereby producing an article having the two permanent magnets 2 bound to each other with the binding layer 6 interposed therebetween. In his binding process, the heating temperature T was of 520° C. and exceeded the liquid phase generating temperature Tm of the brazing filler metal 12 equal to 474° C. and hence, the brazing filler metal 12 was brought into a liquid phase state. In this case, the thickness t of the diffusion area 10 was nearly equal to 20 μm.

For comparison, an epoxy resin-based adhesive (made by Nippon CIBA-GEIGY CO., LTD, under a trade name of ARALDITE) was interposed between end faces of two permanent magnets 2 similar to those described above, and such state was maintained using a preselected jig. Then, the permanent magnets and the adhesive were placed into a drying oven and subjected to a binding process containing a heating step at a heating temperature of 20° C. for a heating time of 60 minutes and a subsequent cooling step, thereby providing an article similar to that described above and having the two permanent magnets 2 joined through the epoxy resin-based adhesive.

The article made using the brazing filler metal 12 and the articles made using the epoxy resin-based adhesive were subjected to a tensile test under heating at 130° C. to provide the results given in Table 6.

TABLE 6 Article Tensile strength (kgf/mm²) Article made using 3.78 the brazing filler metal Article made using the 0.5 adhesive

As is apparent from Table 6, the article 1 made using the brazing filler metal 12 had a high bond strength, as compared with the article made using the epoxy resin-based adhesive.

The permanent magnet 2 containing the rare earth element such as the NdFeB based permanent magnet was poor in wettability to the commercially available adhesive material and for this reason, it was difficult to firmly join the permanent magnets 2 to each other using the commercially available adhesive material.

If the brazing filler metal is used, the permanent magnets 2 can be firmly bound to each other by the brazing filler metal, thereby meeting the demand for a large-sized permanent magnet in a high power motor.

It is required for a motor rotor that the permanent magnet 2 be subjected to a rust-preventing treatment. For this article 1 made using the brazing filler metal 12, the chemical constituents in the permanent magnet 2 and the binding layer 6 are similar to each other and hence, the permanent magnet 2 and the binding layer 6 can easily be subjected to a rust-preventing treatment such as an Ni-plating, an Al ion plating and the like.

When the adhesive is used, the article cannot be Ni-plated or the like and hence, the permanent magnet is subjected to an Ni-plating or the like prior to the binding process. However, the binding layer appears as a joint, resulting in a poor appearance of the article.

Example 4

Neodymium (Nd) having a purity of 99.9%, copper (Cu) having a purity of 99.9%, and chromium (Cr)(which is a diffusion area forming element DA) having a purity of 99.9% were weighed, so that an Nd₇₀Cu₂₈Cr₂ alloy was produced. Then, the weighed elements were melted using a vacuum melting furnace, and then subjected to a casting process to produce an ingot having a length of 10 mm, a width of 10 mm and a height of 50 mm. The ingot was subjected to a cutting to provide a thin plate-like brazing filler metal having a length of 10 mm, a width of 10 mm and a thickness of 0.3 mm. The liquid phase generating temperature of the Nd₇₀Cu₂₈Cr₂ alloy was equal to 538° C., and the longitudinal elastic modulus E thereof was equal to 4,190 kgf/mm².

An NdFeB based permanent magnet 2 (made by Sumitomo Special Metals Co., LTD, under a trade name of NEOMAX-28UH and having a Curie point of 310° C.) having a length of 10 mm, a width of 10 mm and a thickness of 5 mm was selected as one member to be bound, as in Example 1, while a laminate 5 made by laminating cold rolled carbon steel sheets 3 each having a thickness of 0.4 mm together and having a length of 10 mm, a width of 10 mm and a height of 15 mm was selected as the other member to be bound, as in Example 1.

As shown in FIG. 5, a brazing filler metal 12 was superposed on the upwardly facing binding surface b of one laminate 5. A permanent magnet 2 with one binding surface a facing downwardly was superposed on the brazing filler metal 12. Another brazing filler metal 12 was superposed on the other upwardly facing binding surface a of the permanent magnet 2, and another laminate 5 with its binding surface b facing downwardly was superposed on the brazing filler metal 12, thereby fabricating a superpositioned block. Ten superpositioned blocks in total were fabricated in the same manner. Then, the superpositioned blocks were placed into a vacuum heating furnace and subjected to a binding process including a heating step at a heating temperature T equal to 580° C. for a heating time h equal to 20 minutes and a subsequent cooling step, thereby producing ten sandwich structures A having both of the laminates 5 bound to the permanent magnet 2 with the binding layers 6 interposed therebetween, as shown in FIG. 6. Each of the sandwich structures A comprises the two articles 1 commonly associated with the single permanent magnet 2. In this binding process, the heating temperature T is equal to 580° C. and exceeds the liquid phase generating temperature Tm of the brazing filler metal of 538° C. and therefore, the brazing filler metal 12 is brought into a liquid phase. In this case, no fracture was produced in the permanent magnet 2, because the longitudinal elastic modulus E of the brazing filler metal, i.e., the Nd₇₀Cu₂₈Cr₂ alloy is equal to 4,190 kgf/mm² and thus is ≦10,000 kgf/mm².

FIG. 15 shows the results of a line analysis for the sandwich structure A. As is apparent from FIG. 15, a gradient of concentration of Cr atom exists in the binding area between the permanent magnet 2 and the binding layer 6 and in the binding area between the laminate 5 and the binding layer C, and from this, it can be seen that the active mutual diffusion occurred. In this case, the thickness t of the diffusion area 10 on the side of the permanent magnet 2 is nearly equal to 30 μm, and the thickness t of the diffusion area 11 on the side of the laminate 5 is nearly equal to 40 μm.

The ten sandwich structures A were subjected to a tensile test at room temperature, thereby providing the results given in Table 7. For comparison, data (see Table 3) for the sandwich structure A in Example 1 are also given in Table 7.

TABLE 7 Thickness t of defusion Tensile strength area (μm) (kgf/mm²) On the side Sandwich of permanent On the side Room Standard structure: magnet of laminate temperature deviation made by 30 40 4.0 0.4 using brazing filler metal Nd₇₀Cu₂₈Cr₂ made by 10 20 3.1 0.4 using brazing filler metal Nd₇₀Cu₃₀

As is apparent from Table 7, it can be seen that when the brazing filler metal Nd₇₀Cu₂₈Cr₂ is used, the thickness t of the diffusion areas 10, 11 is increased, as compared with when the brazing filler metal Nd₇₀Cu₃₀ is used, and as a result, the bond strength is enhanced.

For the purpose of examining the relationship between the Cr content and the bond strength, various brazing filler metals made of Nd—Cu—Cr based alloys having different Cr contents were produced in the same manner as that described above.

Table 8 shows the composition, the liquid phase generating temperature Tm and the longitudinal elastic modulus E for examples 1 to 4 of the brazing filler metals.

TABLE 8 Liquid phase Longitudinal Brazing Composition generating elastic filler (atom %) temperature Tm modulus metal Nd Cu Cr (° C.) (kgf/mm²) Example 1 70 29 1 529 4100 Example 2 70 26.5 3.5 550 4300 Example 3 70 25 5 590 4500 Example 4 70 24.5 5.5 600 4580

Then, using the examples 1 to 4 of the brazing filler metals, a plurality of sandwich structures A corresponding to the examples 1 to 4 of the brazing filler metals were fabricated and then subjected to a tensile test at room temperature.

Table 9 shows the heating temperature T and the heating time h in the binding process, the thickness t of the diffusion areas 10 and 11 and the tensile strength for the examples 1 to 4 of the brazing filler metals.

TABLE 9 Thickness t diffusion area (μm) On the Heating Heating side of On the Tensile Sandwich temperature time h permanent side of strength structure T (° C.) (minute) magnet laminate (kgf/mm²) Example 1 590 20 20 25 3.9 Example 2 580 20 33 42 3.9 Example 3 600 20 30 40 3.6 Example 4 620 20 10 10 0.4

FIG. 16 is a graph illustrating the relationship between the Cr content and the tensile strength taken from Tables 8 and 9. In FIG. 16, points (1) to (4) correspond to the examples 1 to 4 of the sandwich structures A, respectively. In addition, a point (5) corresponds to the sandwich structure A made using the brazing filler metal made of the Nd₇₀Cu₂₈Cr₂, as given in Table 7, and a point (6) corresponds to the sandwich structure A made using the brazing filler metal made of the Nd₇₀Cu₃₀, as also given in Table 7.

As is apparent from FIG. 16, if the brazing filler metal having the Cr content set at a value ≦5 atom % is used as are the examples 1 to 3 and 5, the thickness t of the diffusion area 10, 11 can be increased to enhance the bond strength.

Even when vanadium (V) is used as the diffusion area forming element, an effect substantially similar to that provided when chromium (Cr) is used is achieved. As one example, data for a brazing filler metal made of an Nd₇₀Cu₂₈V₂ alloy are given below. Liquid phase generating temperature Tm of 540° C.; longitudinal elastic modulus E of 4,200 kgf/mm²; heating temperature T in the binding process of 580° C.; heating time in the binding process of 20 minutes; thickness t of the diffusion areas 10, 11 of 20 μm on the side of the permanent magnet and 25 μm on the side of laminate; and tensile strength of the sandwich structure of 3.6 kgf/mm².

FIGS. 17 and 18 show a motor rotor as an article 1. The rotor 1 includes a plurality of NdFeB based permanent magnets 2 bound to a rotor body 14 comprised of a laminated core using an Nd₇₀Cu₃₀ alloy. A spline shaft portion 16 of a rotary shaft 15 is press-fitted into a spline hole 17 in the rotor body 14, and one end of the rotor body 14 is bound to the rotary shaft 15 through a weld zone 18.

In this rotor 1, the permanent magnets 2 did not fall off the rotor body 14, even when the rotor 1 was rotated at a speed equal to or higher than 10,000 rpm.

Example 5

Neodymium (Nd) having a purity of 99.9% and copper (Cu) having a purity of 99.9% were weighed, so that an Nd₇₀Cu₃₀ alloy was produced. Then, the weighed elements were melted using a vacuum melting furnace, and then subjected to a casting process to produce an ingot.

About 200 g of a starting material was taken from the ingot made of the Nd₇₀Cu₃₀ alloy and melted by a high frequency within a quartz nozzle to prepare a molten metal. Then, the molten metal was ejected through a slit in the quartz nozzle onto an outer peripheral surface of a cooling roll made of copper (Cu) and rotating at a high speed below the slit under an argon gas pressure and then subjected to ultra-quenching, thereby providing a thin foil having a width of 30 mm and a thickness of 90 μm.

In this case, the producing conditions were as follows. The inside diameter of the quartz nozzle was 40 mm; the size of the slit was 0.25 mm wide and 30 mm long; the argon gas pressure was 1.0 kgf/cm²; the temperature of the molten metal was 670° C.; the distance between the slit and the cooling roll was 1.0 mm; the peripheral speed of the cooling roll was 20 m/sec; and the cooling rate of the molten metal was about 10⁵ K/sec.

FIG. 19 shows the results of an X-ray diffraction for the thin foil made of the Nd₇₀Cu₃₀ alloy. In the thin foil, a wide halo pattern was observed at 2 θ=31° and from this, it was determined that the thin foil had an amorphous single-layer structure and therefore, had a volume fraction Vf of an amorphous phase equal to 100%. In addition, the thin foil had a high toughness and was capable of being contact-bent through 180°.

In a binding operation, a foil-like amorphous brazing filler metal 12 having a length of 10 mm, a width of 10 mm and a thickness of 90 μm was cut off the amorphous thin foil.

An NdFeB based permanent magnet (made by Sumitomo Special Metals Co., LTD, under a trade name of NEOMAX-28UH and having a Curie point of 310° C.) having a length of 10 mm, a width of 10 mm and a thickness of 4 mm was selected as one member to be bound, while a rectangular parallelepiped-shaped laminate 5 made by laminating cold rolled carbon steel sheets 3 having a thickness of 0.3 mm together and having a length of 10 mm, a width of 10 mm and a height of 15 mm was selected as the other member to be bound. In this case, a crimping means 4 was used for binding the steel plates.

As shown in FIG. 5, a brazing filler metal 12 was superposed on an upwardly facing binding surface b of one laminate 5. A permanent magnet 2 with one binding surface a facing downwardly was superposed on the brazing filler metal 12. Another brazing filler metal 12 was superposed on the other upwardly facing binding surface a of the permanent magnet 2, and another laminate 5 with its binding surface b facing downwardly was superposed on the brazing filler metal 12, thereby fabricating a superpositioned block. Then, the superpositioned block was placed into a vacuum heating furnace and subjected to a heating step at a heating temperature T of 530° C. for a heating time of 20 minutes and then to a cooling step, thereby providing a sandwich structure A having both of the laminates 5 bound to the permanent magnet 2 bound to each other with the crystalline binding layer 6 interposed therebetween. In this binding process, the brazing filler metal 12 was brought into a liquid phase, because the heating temperature T was equal to 530° C. and exceeded a eutectic point of 520° C. shown in FIG. 4.

For comparison, the ingot was subjected to a cutting by a micro-cutter to fabricate a thin plate-like crystalline brazing filler metal made of an Nd₇₀Cu₃₀ alloy and having a length of 10 mm, a width of 10 mm and a thickness of 0.25 mm (a limit thickness available by the micro-cutter). Using this brazing filler metal, a sandwich structure having the same configuration as that of the sandwich structure shown in FIG. 6 was produced in the same manner as that described above.

A plurality of the sandwich structures A made using the amorphous brazing filler metal 12 and a plurality of the sandwich structures made using the crystalline brazing filler metal was subjected to a tensile test at room temperature and under heating at 150° C., thereby providing the results given in Table 10. Data (see Table 3) for the sandwich structure B in Example 1 are also given in Table 10 for comparison.

TABLE 10 Thickness t of diffusion Tensile strength area (μm) (kgf/mm²) Sandwich On the side of On the side Room structure: permanent magnet of laminate temperature 150° C. made by 15 25 4.5 4.4 using amorphous brazing filler metal made by 10 20 3.1 3.0 using crystalline brazing filler metal made by — — 1.5 0.5 using adhesive

As is apparent from Table 10, the sandwich structure A made using the amorphous brazing filler metal 12 had a high bond strength at room temperature and under heating at 150° C., as compared with the sandwich structure made using the adhesive. The bond strength of the sandwich structure A was little changed in both of such environments, and the change thereof was small.

The bond strength of the sandwich structure A made using the amorphous brazing filler metal 12 was higher at both the temperatures than that of the sandwich structure A made using the crystalline brazing filler metal. This is attributable to the fact that the amorphous brazing filler metal had an excellent oxidation resistance, was squeezed out only in a very small amount, and further has a homogeneous composition.

Further, the thickness of the amorphous brazing filler metal was ⅓ of the thickness of the crystalline brazing filler metal and therefore, the amorphous brazing filler metal was squeezed out only in a small amount, and a post-treatment after the binding process was easily carried out.

Example 6

Mixed phase brazing filler metals and amorphous brazing filler metals having various compositions were produced using a single-roller melt-spinning technique similar to that in Example 5, and then subjected to a binding process similar to that in Example 5, thereby providing various sandwich structures. Then, these sandwich structures were subjected to a tensile test similar to that in Example 5. Various crystalline brazing filler metals were fabricated from ingots having various compositions in the same manner as in Example 5 for comparison.

Table 11 shows the compositions of the various brazing filler metals, binding conditions and the bond strength of the sandwich structures.

TABLE 11 Thickness t of Brazing filler metal diffusion area Tensile strength of Contact- Joining process (μm) sandwich structure bending Heating Heating On the On the (kgf/mm²) Example Metallographic through temperature time h side of side of Room No. Composition structure 180° (° C.) (minute) magnet laminate temperature 150° C. 1 Nd₇₅Cu₂₅ amorphous phase impossible 530 20 15 25 4.1 4.0 (Vf = 75%) crystalline phase 1a crystalline phase impossible 10 20 3.1 3.0 2 La₇₁Cu₂₉ amorphous phase possible 485 20 15 22 4.3 4.1 2a crystalline phase impossible 10 15 3.0 2.9 3 Ce₇₂Cu₂₈ amorphous phase possible 435 20 15 20 4.2 3.8 3a crystalline phase impossible 10 15 3.0 2.7

In Table 11, the example 1 of the brazing filler metal is a mixed-phase brazing filler metal having a metallographic structure containing mixed phases of an amorphous phase and a crystalline phase, but is homogeneous in composition, as compared with the example 1a of the crystalline brazing filler metal, because it has a volume fraction Vf of the amorphous phase of 75% which satisfies the condition that Vf≧50%. As a result, the bond strength of the sandwich structure produced from the example 1 is increased. Both of the examples 2 and 3 of the brazing filler metals are amorphous brazing filler metals, while both of the examples 2a and 3a of the brazing filler metals are crystalline brazing filler metals. Even in this case, it can be seen that if the examples 2 and 3 of the amorphous brazing filler metals are used, satisfactory results are obtained.

Example 7

This Example will be described for a brazing filler metal made of Nd—Cu—AE based alloy (a rare earth element-based alloy) used in a binding process.

Neodymium (Nd) having a purity of 99.9%, copper (Cu) having a purity of 99.9%, and carbon (C) having a purity of 99.9% were weighed, so that an Nd₇₀Cu₂₀C₁₀ alloy (each of numerical values is by atom % and as forth) was produced. Then, the weighed elements were melted using a vacuum melting furnace, and then subjected to a casting process to produce an ingot.

About 50 g of a starting material was taken from the ingot and melted by a high frequency in a quartz nozzle to prepare a molten metal. Then, the molten metal was ejected through a slit in the quartz nozzle onto an outer peripheral surface of a copper cooling roll rotating at a high speed below the slit under an argon gas pressure and then ultra-quenching, thereby producing a thin foil made of the Nd₇₀Cu₂₀C₁₀ alloy having a width of 30 mm and a thickness of 20 μm. This thin foil was homogeneous and good in continuity and therefore, the foil-forming property of the alloy having the above-described composition was good.

Producing conditions in this case were as follows. The inside diameter of the quartz nozzle was 40 mm; the slit had a width of 0.25 mm and a length of 30 mm; the argon gas pressure was 1.0 kgf/cm²; the temperature of the molten metal was 800° C.; the distance between the slit and the cooling roll was 1.0 mm; the peripheral speed of the cooling roll was 33 m/sec; and the cooling rate of the molten metal was about 10⁵ K/sec.

FIG. 20 shows the results of an X-ray diffraction for the thin foil. In this thin foil, a wide halo pattern was observed at 2θ≅31° and from this, it was determined that the metallographic structure of the thin foil was an amorphous single-layer structure (a volume fraction Vf of an amorphous phase being equal to 100%). The crystallizing temperature Tx was 190.7° C. In addition, the liquid phase generating temperature of the thin foil was 540.3° C., and the fusibilization was achieved. Further, the thin foil had a high toughness and hence, was capable of being contact-bent through 180°. Yet further, the thin foil was not discolored and had an excellent oxidation resistance. Further, thin foils having varied thicknesses of 20 μm to 400 μm were produced in the same conditions except that only the peripheral speed of the cooling roller was varied, thereby determining a critical thickness of the thin foil at which an amorphous single-phase structure was formed. The result showed that the critical thickness was 270 μm. The longitudinal elastic modulus E of the Nd₇₀Cu₂₀C₁₀ alloy was 4300 kgf/mm².

Then, the thin foil having a thickness of 100 μm was subjected to a punching, thereby fabricating a foil-like amorphous brazing filler metal 12 having a length of 10 mm, a width of 10 mm. This brazing filler metal was subjected to a binding operation in a following manner.

An NdFeB based permanent magnet (made by Sumitomo Special Metals Co., LTD, under a trade name of NEOMAX-28UH and having a Curie point of 310° C.) having a length of 10 mm, a width of 10 mm and a thickness of 3 mm was selected as one member to be bound, while a rectangular parallelepiped-shaped laminate 5 made by laminating cold rolled carbon steel sheets 3 having a thickness of 0.3 mm together and having a length of 10 mm, a width of 10 mm and a height of 15 mm was selected as the other member to be bound. In this case, a crimping means 4 was used for binding the steel plates.

As shown in FIG. 5, a brazing filler metal 12 was superposed on an upwardly facing binding surface b of one laminate 5. A permanent magnet 2 with one binding surface a facing downwardly was superposed on the brazing filler metal 12. Another brazing filler metal 12 was superposed on the other upwardly facing binding surface a of the permanent magnet 2, and another laminate 5 with its binding surface b facing downwardly was superposed on the brazing filler metal 12, thereby fabricating a superpositioned block. Then, the superpositioned block was placed into a vacuum heating furnace and subjected to a heating step at heating temperature T of 550° C. for a heating time h of 15 minutes and then to a cooling step, thereby providing a sandwich structure A having both of the laminates 5 bound to the permanent magnet 2 with the amorphous binding layer 6 interposed therebetween. In this binding process, the brazing filler metal 12 was brought into a solid/liquid coexisting state, because the heating temperature T was equal to 550° C. and exceeded the liquid phase generating temperature Tm of 540.3° C.

A plurality of the sandwich structures A were subjected to tensile test at room temperature, thereby providing the results shown in Table 12. Data (see Table 3) for the sandwich structure B in Example 1 are also given in Table 12 for comparison.

TABLE 12 Thickness t of diffusion area (μm) Tensile Sandwich On the side On the side strength structure of magnet of laminate (kgf/mm²) made by using 13 22 4.2 amorphous brazing filler metal made by using — — 1.5 adheseve

As can be seen from Table 12, the sandwich structure A made using the brazing filler metal 12 had a high bond strength, as compared with the sandwich structure made using the adhesive.

Then, amorphous thin foils having various compositions were produced using a single-roller melt-spinning technique similar to that described above and then subjected to an X-ray diffraction and further to a binding operation similar to that described above. Sandwich structures were fabricated from the thin foils and then subjected to a tensile test. Tables 13 and 14 show the results. In this case, the heating temperature T in the binding operation was likewise set at 550° C. and therefore, the brazing filler metal was brought into a liquid phase state or a solid/liquid coexisting state. Data for the thin foil made of the Nd₇₀Cu₂₀C₁₀ alloy are also given as example 20 in Tables 13 and 14.

TABLE 13 Composition : Nd₇₀Cu₂₀AE₁₀ Liquid phase Longitudinal Example generating Crystallizing Critical elastic No. of temperature temperature Tx thickness modulus thin foil AE Tm (° C.) (° C.) (μm) (kgf/mm²) 1 Al 473.9 144.8 250 4000 2 Ga 422.4 126.5 250 4150 3 Ag 436.2 107.5 75 4200 4 Co 481.9 150.4 150 4500 5 Pb 491.8 127.4 70 4000 6 Au 501.6 113.6 100 3900 7 Sn 512.5 110.5 80 3950 8 Zn 515.2 120.1 120 4200 9 Si 520.7 208.7 100 4000 10 In 521.7 109.5 90 3900 11 P 521.9 119.8 250 4100 12 Ni 522.4 174.3 160 3950 13 Ge 522.5 198.3 150 4100 14 B 523.9 119.5 75 4200 15 Rh 525.1 133.7 110 4300 16 Pt 527.9 206.2 300 4400 17 Pd 530.2 211.4 300 4450 18 Bi 531.6 135.9 70 4060 19 Sb 535.2 146.3 70 3950 20 C 540.3 190.7 270 4080 21 Fe 501.1 117.6 120 4300 22 Os 529.3 186.1 60 4150 23 Ru 531.8 140.2 100 4250 24 Ir 538.1 141.9 75 4100

TABLE 14 Thickness t of diffusion area (μm) On the side of Tensile Example No. permanent On the side of strength of thin foil magnet laminate (kgf/mm²) 1 15 25 4.4 2 10 22 4.6 3 13 24 4.2 4 17 20 4.5 5 18 28 4.2 6 15 20 4.3 7 18 28 4.2 8 18 30 4.3 9 10 18 4.4 10 12 20 4.3 11 10 20 4.3 12 18 30 4.4 13 11 22 4.3 14 10 25 4.4 15 13 20 4.4 16 15 25 4.4 17 15 25 4.3 18 10 23 4.1 19 10 20 4.2 20 20 32 4.2 21 20 30 4.3 22 10 20 4.1 23 15 23 4.3 24 15 25 4.2

In table 13, the highest value of the liquid phase generating temperature Tm of the thin foil and thus the rare earth element-based alloy is 540.3° C. in the example 20. From this, it can be seen that the fusibilization was achieved in each of the examples.

The crystallizing temperature Tx is a temperature at which the amorphous phase is transformed to the crystalline phase, and therefore, the higher the crystallizing temperature Tx, the more stably the amorphous phase exists.

Further, the critical thickness indicates the degree of the amorphous phase forming ability of the rare earth element-based alloy. As the critical thickness is larger, the amorphous phase forming ability is higher and therefore, it is possible to produce an amorphous thin foil with a stability.

Example 8

This Example will be described for a brazing filler metal made of Nd—Cu—Al based alloy (a rare earth element-based alloy) being used in a binding process.

Neodymium (Nd) having a purity of 99.9%, copper (Cu) having a purity of 99.9%, and aluminum (Al) having a purity of 99.9% were weighed, so that an Nd—Cu—Al alloy having a predetermined composition was produced. Then, the weighed elements were melted using a vacuum melting furnace, and then subjected to a casting process to produce an ingot.

About 50 g of a starting material was taken from the ingot and melted by a high frequency in a quartz nozzle to prepare a molten metal. Then, the molten metal was ejected through a slit in the quartz nozzle onto an outer peripheral surface of a copper cooling roll rotating at a high speed below the slit under an argon gas pressure and then ultra-quenching, thereby producing a thin foil made of the Nd—Cu—Al alloy and having a width of 30 mm and a thickness of 100 μm.

Producing conditions in this case were as follows. The inside diameter of the quartz nozzle was 40 mm; the slit had a width of 0.25 mm and a length of 30 mm; the argon gas pressure was 1.0 kgf/cm²; the temperature of the molten metal was 670° C.; the distance between the slit and the cooling roll was 1.0 mm; the peripheral speed of the cooling roll was 13 m/sec; and the cooling rate of the molten metal was about 10⁵ K/sec.

The thin foils were examined for the melting temperature at which 80% or more of the thin foil was melted into a liquid phase, the metallographic structure, the thin foil forming property, the oxidation resistance and the longitudinal elastic modulus E, thereby providing the results given in Table 15. At the column “metallographic structure”, “amo” indicates an amorphous single-phase structure, and “amo+cry” indicates a mixed phase structure containing an amorphous phase amo and a crystalline phase cry.

TABLE 15 Example Chemical Longitudinal No. constituent Melting Thin foil elastic of thin (by atom %) temperature Metallographic forming Oxidation modulus E foil Nd Cu Al (° C.) structure property resistance (kgf/mm²)  1 60 30 10 613.4 amo good good 4200  2 60 20 20 666.3 amo possible good 4250  3 65 30  5 609.1 amo good good 4150  4 68 30  2 513.6 amo good good 4000  5 68 28  4 509.5 amo good good 4020  6 68 26  6 507.8 amo good good 4050  7 68 24  8 511.7 amo good good 4070  8 70 28  2 506.7 amo good good 3950  9 70 27  3 506.6 amo good good 3960 10 70 26  4 504.4 amo good good 3980 11 70 25  5 506.7 amo good good 4000 12 70 24  6 508.4 amo good good 4020 13 70 22  8 509.1 amo good good 4050 14 70 20 10 576.5 amo good good 4070 15 72 26  2 506.9 amo + cry possible possible 4100 16 72 24  4 510.5 amo + cry possible possible 4120

As is apparent from Table 15, in the thin foil and thus in the Nd—Cu—Al alloy, the Nd content may be set in a range of 68 atom %≦Nd≦70 atom %; the Cu content may be set in a range of 22 atom %≦Cu≦30 atom %; and the Al content may be set in a range of 2 atom %≦Al≦8 atom % as in the examples 4 to 13 shown in FIG. 21, in order to provide the fusibilization, to achieve the non-crystallization and further to improve the thin foil forming property and the oxidation resistance.

Then, the example 11 of the amorphous thin foil made of the Nd₇₀Cu₂₅Al₅ was subjected to a punching, thereby fabricating a foil-like amorphous brazing filler metal 12 having a length of 10 mm, a width of 10 mm and a thickness of 100 μm. Using this brazing filler metal 12, a binding operation was carried out in the following manner.

An NdFeB based permanent magnet (made by Sumitomo Special Metals Co., LTD, under a trade name of NEOMAX-28UH and having a Curie point of 310° C.) having a length of 10 mm, a width of 10 mm and a thickness of 4 mm was selected as one member to be bound, as in Example 7, while a rectangular parallelepiped-shaped laminate 5 made by laminating cold rolled carbon steel sheets 3 having a thickness of 0.3 mm together and having a length of 10 mm, a width of 10 mm and a height of 15 mm was selected as the other member to be bound, as in Example 7.

As shown in FIG. 5, a brazing filler metal 12 was superposed on an upwardly facing binding surface b of one laminate 5. A permanent magnet 2 with one binding surface a facing downwardly was superposed on the brazing filler metal 12. Another brazing filler metal 12 was superposed on the other upwardly facing binding surface a of the permanent magnet 2, and another laminate 5 with its binding surface b facing downwardly was superposed on the brazing filler metal 12, thereby fabricating a superpositioned block. Then, the superpositioned block was placed into a vacuum heating furnace and subjected to a heating step at heating temperature T of 510° C. for a heating time h of 30 minutes and then to a cooling step, thereby providing a sandwich structure A having both of the laminates 5 bound to the permanent magnet 2 with the amorphous binding layer 6 interposed therebetween. In this binding process, the brazing filler metal 12 was brought into a liquid phase state, because the heating temperature T was equal to 510° C. and exceeded the melting temperature Tm of the brazing filler metal 12 of 506.7° C.

For comparison, an amorphous foil made of an Nd₇₀Cu₃₀ alloy and having a thickness of 100 μm was subjected to a punching, thereby fabricating a brazing filler metal having a length of 10 mm and a width of 10 mm. Using this brazing filler metal, a sandwich structure having the same configuration as the sandwich structure A shown in FIG. 6 was produced in the same manner as described above. In this case, the heating temperature T was set at 530° C., because the Nd₇₀Cu₃₀ alloy was a eutectic alloy and had a eutectic point of 520° C.

The sandwich structure A made using the brazing filler metal 12 made of the Nd₇₀Cu₂₅Al₅ alloy and the sandwich structure made using the Nd₇₀Cu₃₀ alloy were subjected to a tensile test at room temperature and under heating at 150° C., thereby providing the results given in Table 16.

TABLE 16 Thickness t of diffusion area Tensile strength (μm) (kgf/mm²) On the Heating On the side of Room Sandwich temperat- side of lam- temp- structure ure T (° C.) magnet inate erature 150° C. made using the 510 18 28 4.4 4.4 brazing filler metal of Nd₇₀Cu₂₅Al₅ alloy made using 530 16 20 4.5 4.4 Nd₇₀Cu₃₀ alloy

As can be seen from Table 16, the sandwich structure A made using the brazing filler metal 12 made of the Nd₇₀Cu₂₅Al₅ alloy had a bond strength substantially equivalent to that of the sandwich structure made using the brazing filler metal made of the Nd₇₀Cu₃₀ alloy at room temperature and under heating at 150° C., notwithstanding that the heating temperature T in the binding process was set at 510° C. and was 20° C. lower than 530° C. which was the heating temperature for the brazing filler metal made of the Nd₇₀Cu₃₀ alloy. This was attributable to the fusibilization was provided in the Nd₇₀Cu₂₅Al₅ alloy.

Example 9

This Example will be described for a brazing filler metal made of RE—Cu—Al based alloy (a rare earth element-based alloy) being used in a binding process.

Praseodymium (Pr) having a purity of 99.9%, copper (Cu) having a purity of 99.9%, and aluminum (Al) having a purity of 99.9% were weighed, so that a Pr₆₈Cu₂₇Al₅ alloy was produced. Then, the weighed elements were melted using a vacuum melting furnace, and then subjected to a casting process to produce an ingot.

About 50 g of a starting material was taken from the ingot and melted by a high frequency in a quartz nozzle to prepare a molten metal. Then, the molten metal was ejected through a slit in the quartz nozzle onto an outer peripheral surface of a copper cooling roll rotating at a high speed below the slit under an argon gas pressure and then ultra-quenching, thereby producing a thin foil made of the Pr₆₈Cu₂₇Al₅ alloy and having a width of 30 mm and a thickness of 100 μm.

Producing conditions in this case were as follows. The inside diameter of the quartz nozzle was 40 mm; the slit had a width of 0.25 mm and a length of 30 mm; the argon gas pressure was 1.0 kgf/cm²; the temperature of the molten metal was 580° C.; the distance between the slit and the cooling roll was 1.0 mm; the peripheral speed of the cooling roll was 13 m/sec; and the cooling rate of the molten metal was about 10⁵ K/sec.

FIG. 22 shows the results of an X-ray diffraction for the thin foil. In this thin foil, a wide halo pattern was observed at 2θ=32° C. and from this, it was determined that the metallographic structure of the thin foil was an amorphous single-layer structure. In addition, the thin foil had a high toughness and was capable of being contact-bent through 180°. Further, the longitudinal elastic modulus E of the thin foil was 4100 kgf/mm².

Amorphous thin foils having various compositions were produced in the same manner as that described above, except that the amount of aluminum incorporated was 5 atom % (constant), and the type of the rare earth element RE and the amount of rare earth element RE incorporated were varied. The thickness of the thin foils was set at 100 μm. The thin foil 5 were examined for the liquid phase generating temperature Tm and the longitudinal elastic modulus E, thereby providing the results given in Table 17.

TABLE 17 Liquid phase Longitudinal Example generating elastic No. of temperature modulus E thin foil Composition Tm (° C.) (kgf/mm²) 1 Ce₇₂Cu₂₃Al₅ 386.3 4100 2 Mm₇₂Cu₂₃Al₅(Mm:58 atom % 385.4 4200 of Ce, 24 atom % of La, 13 atom % of Nd and 5 atom % of Pr) 3 Pr₆₈Cu₂₇Al₅ 427.3 4100 4 La₇₁Cu₂₄Al₅ 414.7 4140 5 Sm₇₁Cu₂₄Al₅ 565.6 4000

As is apparent from Table 17, in each of the thin foils and thus the RE—CU—Al based alloys, the liquid phase generating temperature Tm was in a range of 380° C.≦Tm≦600° C., and the fusibilization was provided. In order to achieve the fusibilization, the content of the rare earth element RE may be set in a range of 60 atom %≦RE≦80 atom %; the Cu content may be set in a range of 20 atom %≦Cu≦40 atom %; and the Al content may be set in a range of Al≦10 atom %, and at least one element selected from the group consisting of La, Ce, Pr, Nd and Sm may be used as the rare earth element RE. The thin foil containing Nd is given as example 11 in Table 15.

Then, the example 3 of the amorphous thin foil made of the Pr₆₈Cu₂₇Al₅ alloy was subjected to a punching, thereby fabricating a foil-like amorphous brazing filler metal having a length of 10 mm and a width of 10 mm and having a thickness of 100μ. Using this brazing filler metal, a binding operation was carried out in the following manner.

An NdFeB based permanent magnet (made by Sumitomo Special Metals Co., LTD, under a trade name of NEOMAX-28UH and having a Curie point of 310° C.) having a length of 10 mm, a width of 10 mm and a thickness of 4 mm was selected as one member to be bound, while a rectangular parallelepiped-shaped laminate 5 made by laminating cold rolled carbon steel sheets 3 having a thickness of 0.4 mm together and having a length of 10 mm, a width of 10 mm and a height of 15 mm was selected as the other member to be bound.

As shown in FIG. 5, a brazing filler metal 12 was superposed on an upwardly facing binding surface b of one laminate 5. A permanent magnet 2 with one binding surfaces a facing downwardly was superposed on the brazing filler metal 12. Another brazing filler metal 12 was superposed on the other upwardly facing binding surface a of the permanent magnet 2, and another laminate 5 with its binding surface b facing downwardly was superposed on the brazing filler metal 12, thereby fabricating a superpositioned block. Then, the superpositioned block was placed into a vacuum heating furnace and subjected to a heating step at heating temperature T of 440° C. for a heating time h of 15 minutes and then to a cooling step, thereby providing a sandwich structure A having each of the laminates 5 and the permanent magnet 2 bound to each other with the amorphous binding layer 6 interposed therebetween. In this binding process, the brazing filler metal 12 was brought into a liquid phase state, because the heating temperature T was 440° C. and exceeded the liquid phase generating temperature Tm of 427.3° C.

For comparison, examples 2 to 6 of sandwich structures were produced in the same manner as that described above, except that in place of the permanent magnet 2, various members to be bound having the same size as the permanent magnet 2 were used. Further, an example 7 of a sandwich structure was produced in the same manner, except that in place of the permanent magnet 2, an FeSiB based amorphous thin piece (made by Alide Co. under a trade name of 2605S-2) 10 mm long and 20 mm wide and with a thickness of 26 μm and a brazing filler metal 12 having a thickness of 20 μm was used.

The examples 1 to 7 of the sandwich structures A were subjected to a tensile test at room temperature, thereby providing the results given in Table 18.

TABLE 18 Thickness of diffusion area t (μm) Example On No. of the side On the Tensile sandwich of the side of strength structure Type of member member laminate (kgf/mm²) 1 NdFeB based permanent 14 24 4.4 magnet 2 member of steel (JIS 20 26 15.2 SCM432) 3 member of stainless steel 25 25 14.6 (JIS SUS416) 4 member of Ni alloy 10 25 12.5 (Hastelloy B) 5 member of copper 10 23 17.3 (oxygen-free copper) 6 TbDyFe based super- 15 22 3.9 magnetostrictive member (made by Etrema Products Co., Inc. under a trade name of Terfenol D) 7 FeSiB based amorphous 7 7 14.1 thin foil

For each of the examples 2 to 5 and 7 in Table 8, a fracture was produced in the binding layer 6, but for the example 1, the permanent magnet 2 was fractured, and for the example 6, the super-magnetostrictive member was fractured. It can be seen from Table 18 that a large bond strength is obtained, notwithstanding that the heating temperature was as low as 440° C.

The member made of a carbon steel or a stainless steel is hardened by a quenching, but when this member is heated to 750° C. to 900° C., it is annealed and softened. For the rare earth element-based super-magnetostrictive member, when it is heated to 900° C. or more, a disadvantage occurs that the magnetic characteristic thereof is degraded, because the metallographic structure is varied. However, such disadvantage can be avoided, because the heating temperature T is low in the binding operation using the above-described brazing filler metal 12.

Then, a brazing filler metal was fabricated in the same manner as described above from the example 2 of the amorphous thin foil made of the Mm₇₂Cu₂₃Al₅. Using this brazing filler metal, a binding operation similar to that described above was carried out, thereby producing examples 1 to 6 of sandwich structures A similar to those described above. An example 7 of a similar sandwich structure A was also produced in the same manner. The heating temperature T was set at 420° C.

Table 19 shows the results of a tensile strength for the examples 1 to 7 of the sandwich structures A.

TABLE 19 Thickness of diffusion area t (μm) Example On No. of the side On the Tensile sandwich of the side of strength structure Type of member member laminate (kgf/mm²) 1 NdFeB based permanent 15 22 4.4 magnet 2 member of steel (JIS 18 24 16.1 SCM432) 3 member of stainless steel 23 25 13.9 (JIS SUS416) 4 member of Ni alloy 11 20 12.8 (Hastelloy B) 5 member of copper 10 20 16.5 (oxygen-free copper) 6 TbDyFe based super- 15 25 3.9 magnetostrictive member (made by Etrema Products Co., Inc. under a trade name of Terfenol D) 7 FeSiB based amorphous 6 7 14.2 thin foil

Likewise, for the examples 2 to 5 and 7, a fracture was produced in the binding layer 6, but for the example 1, the permanent magnet 2 was fractured, and for the example 6, the super-magnetostrictive member was fractured. It can be seen from Table 18 that a large bond strength is obtained, notwithstanding that the heating temperature was as low as 420° C. 

What is claimed is:
 1. An article made by joining two metal structural members by a binding layer interposed therebetween comprising, one of said two structural members being made of a permanent magnet including a rare earth element, the other of said two structural members being made of laminated metal sheets superposed one on another, and said binding layer being formed from a brazing filler metal, wherein said two structural members are joined together by heating to a temperature sufficient to cause a liquid phase in said binding layer and by producing mutually diffused areas between said one structural member and said binding layer and between said other structural member and said binding layer, respectively, by utilizing said liquid phase, and wherein said brazing filler metal forming said binding layer contains a rare earth element RE in a content of RE≧50 atom % Cu in a content of 20 atom %≦Cu≦40 atom %, and an alloy element AE in a content of 0 atom %<AE≦20 atom %, said rare earth element RE is at least one element selected from the group consisting of Pr, Nd, La, Ce and Sm, and said alloy element AE is at least one element selected from the group consisting of Fe, Co, Ni, Ag, Al, Ga and Si.
 2. An article according to claim 1, wherein the content of said AE element is in a range of 2 atom %≦AE≦20 atom %.
 3. An article according to claim 1, wherein the thickness t of each of said mutually diffused areas is in a range of t≧5 μm.
 4. An article according to claim 1, wherein the thickness t of each of said mutually diffused areas is in a range of t≧10 μm.
 5. An article according to claim 1, wherein a liquid phase generating temperature Tm of said brazing filler metal is in a range of Tm≦650° C.
 6. An article according to claim 1, wherein said brazing filler metal has a volume fraction Vf of an amorphous phase in range of Vf≧50%.
 7. An article according to claim 1, wherein said brazing filler metal further contains at least one diffusion forming element DA in a concentration of DA≦5 atom %, said diffusion forming element DA being selected from the group consisting of Cr and V.
 8. An article according to claim 1, wherein said brazing filler metal has a longitudinal elastic modulus E in the range of 300 kgf/mm₂≦E≦10,000 kgf/mm₂. 